Collier, 27, was shot to death in his patrol car Thursday night. Authorities say he was killed by Boston Marathon bombing suspects Tamerlan and Dzhokhar Tsarnaev. Tamerlan was killed in a shootout with police, while a wounded Dzhokhar was captured Friday evening.

"Officer Collier didn't just have a job at MIT. He had a life at MIT," said MIT President L. Rafael Reif, one of several people scheduled to speak at the three-hour service.

"In just 15 months, he built a life with us that was rich in friendship and shared adventure," Reif said. "MIT is a place that celebrates passionate curiosity, and Sean Collier fit right in."

Vice President Biden and U.S. Sen. Elizabeth Warren, D-Mass., also spoke at the service. Speaking to Collier's family, Biden said: "My heart goes out to you. I hope you find some solace in what's being said here about your son."

Recalling his own grief from the deaths of his daughter and first wife in a 1972 car crash, Biden said the Colliers would eventually overcome some of their pain.

"You know it's going to be OK when you pass a little league field or hear a song that reminds you of Sean ... when you get a smile to your lips before a tear comes to your eye," he said.

MIT Police Chief John DiFava last saw Collier at about 9:30 p.m. Thursday, just hours before his death. "He gave me that famous grin," DiFava said. "I told him to be safe."

Brandon Russell, a fifth-year graduate student, praised the ceremony. "It was reverent; there were moments of levity. It was lovely and touching,'' he said. "The chance to let out feelings and be with other people who also wanted to cry and had similar relationships and felt the same way, I thought it would help. And it did."

Collier was well-respected by colleagues and superiors, and popular with students. He often went on hikes with the MIT student outing club.

Bruno Faviero, an MIT sophomore covering the memorial service for the school's student newspaper, said he had seen Collier around campus. Collier "always a friendly smile and a warm hello," said Faviero, who saw Collier last Wednesday at a service to honor the three killed and 264 wounded in the bombing attack.

"I never would have thought that two days later he'd be dead,'' Faviero said.

The school set up seating for 15,000 on Briggs Field, a process than took from 11 p.m. Tuesday until 3 a.m. Wednesday. Many of the seats were filled by police officers from throughout Massachusetts and other states.

Meanwhile, Boston's Boylston Street, the site of the twin bombings, reopened to the public Wednesday, although several businesses remain closed.